Field of the Invention
The present invention relates generally to oscillators for use with quartz resonant sensing elements, and more specifically to a means for minimizing component count and power consumption for such oscillators when a mixed frequency output is desired.
State of the Art
Simple quartz crystal oscillators have been built for decades, and are widely used in science and industry. However, in applications wherein power availability and physical space are limited, such as in instrumentation employed in subterranean wellbores and in aerospace components, the additional requirements of buffering, mixing and then filtering the output are burdensome.
Balanced mixers based upon a differential transistor configuration are well known, and are used in communications equipment, as illustrated in Clark, et al, COMMUNICATIONS CIRCUITS ANALYSIS AND DESIGN, Addison-Wesley, (1971) p. 309, and reproduced in the drawings accompanying this application as FIG. 1. In the illustrated circuit, an RF signal is applied to the bases of a differential transistor pair through a transformer. A local oscillator signal is superimposed on the bias current Ik. The collector current of the two transistors contains product terms including sum and difference frequencies. The desired output is selected by the frequency dependent impedances at the collectors. U.S. Pat. No. 4,661,785 to Benjaminson discloses a balanced feedback oscillator employing a differential transistor configuration in a bridge oscillator employing a constant current source. Page 252 of the aforementioned Clark et al. text discloses a differential series oscillator.
Another prior art circuit, a variant of the standard Pierce oscillator, employs two transistors in a differential pair configuration and might be termed a differentially coupled Pierce oscillator. Such a circuit, illustrated by FIG. 2 of the drawings, provides for improved limiting characteristics and better load isolation than a standard Pierce oscillator. The circuit of FIG. 2 has been successfully employed in a quartz crystal resonator pressure transducer. However, the circuit of FIG. 2 does not provide a means for mixing two frequencies. The high frequency, high impedance output must first be buffered, mixed with a second frequency, and then filtered before the desired output is achieved. These functions require additional components and process.
Yet another prior art device illustrative of the state of the art is the Hewlett-Packard Model 2813B Quartz Pressure Probe. This device utilizes thickness-shear mode quartz pressure sensor and reference crystals, employs two active and one bias transistor per oscillator, and an additional five transistors per oscillator for voltage gain, buffering and limiting before the diode mixer stage is entered.